The initial step of AMP degradation is the conversion of AMP to IMP, which is converted to xanthine in two alternative routes, via xanthosine or hypoxanthine. The initial step of GMP degradation is the conversion of GMP to guanosine, which is also degraded to xanthine in two ways, via guanine or xanthosine.

Studies in tea leaves and cultured Catharanthus roseus cells showed that guanosine deaminase activity was much higher than that of guanine deaminase [Ashihara97]. It indicates the degradation pathway of GMP via xanthosine is likely to be the major route. Xanthine is then converted to urate by xanthine dehydrogenase [Hesberg04].